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World's Largest Atom Smasher May Have Detected 'God Particle' (the Higgs boson)

World's Largest Atom Smasher May Have Detected 'God Particle' (the Higgs boson)

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Excerpt: A rumor is floating around the physics community that the world's largest atom smasher may have detected a long-sought subatomic particle called the Higgs boson, also known as the "God particle."
World's Largest Atom Smasher May Have Detected 'God Particle' - FoxNews.com
Collider sees hint of elusive particle's existence - CBS News
samoth in 3, 2, ...

Re: World's Largest Atom Smasher May Have Detected 'God Particle' (the Higgs boson)

This plot was shown yesterday at a seminar which as far as I know was public. It uses 900/pb for the Higgs->ZZ channels combined with around 200/pb for the decay modes favoured at lower masses. Hence the surprising exclusion at higher masses. There are some interesting excesses but nothing of sufficient significance to get really excited about.

This is a draft preview of what will be shown at EPS-HEP in two weeks time although it is possible that more data will be added by then. ATLAS will also contribute results with similar amounts of data. Shortly after EPS-HEP they will combine CMS and ATLAS results to provide a much better plot.

For those of you not familiar with this kind of plot, it shows limits on cross-sections for processes beyond the Higgless standard model with three generation. When it dips below the red line at 1 on the y-axis that means that a Hiiggs is excluded at that mass with 95% confidence. The dotted line shows the expected value given the amount of data collected. This line sinks down as more data is added. The black line is what is observed and is different either because there is new physics or because of statistical fluctuations. So long as it stays inside the green and yelklow bands that mark one or two standard deviations away from the expected, then there is no signal. An excess above this line starts to look interesting but you do expect it to go beyond the bounds somewhere so small deviations are not to be counted on. When it exceeds 5 standard deviations we get excited.

The dotted line going below the red line indicates where an exclusion was expected. As you can see they failed to get an exclusion in the 135 GeV to 200 GeV range. This can either be because there is a Higgs boson lurking or because of statistical fluctuations. For now we have to assume the latter until more data pushes the green and yellow bands down. The exclusion at higher masses was not expected yet and is also due to statistical fluctuations, but it still counts. There are interesting excesses around 115 GeV, 130 GeV and 210 GeV, but they need more data to make these interesting. We may see more data in the next two weeks either from other CMS channels or from ATLAS, so it could get a lot more exciting at EPS-HEP.

The latest technical stop at the LHC has ended and cryogenics are back on time to restart physics (well done). During the next 6 week run they will quickly get back to 1380 bunch fills and then slowly increase luminosity further using bunch intensity and emittance increments. How far they get will depend on how smooth the runs are but luminosities up to 5/nb/s are possible given favorable conditions. That means anything from 1.5/fb to 3.5/fb could be delivered before the next technical stop. There will be a further run of eight weeks after that. My prediction of 10/fb for 2011 is no longer looking so over-optimistic, but the vagaries of the LHC systems will have their say.

Re: World's Largest Atom Smasher May Have Detected 'God Particle' (the Higgs boson)

A Historic Day? Or Just an Important One?
July 22, 2011. This was no ordinary day.

It has been a sad historic day in Europe, with the murders in Oslo. This tragedy obviously trumps the news from a particle physics conference. Our hearts go out to those suffering in Norway.

But the news story from Grenoble is a big one, and it needs telling before I start to forget it.

I came to this conference fully expecting that there would be a significant number of controversial results from the Tevatron, and virtually none from the LHC except in response to the Tevatron. (Here’s why I expected that.) This was ok by me. I expected to learn a great deal anyway.

Of course I also knew there would be an update from the Tevatron and LHC experiments on the search for the Standard Model Higgs particle. This was also something I expected to be somewhat interesting.
The Standard Model Higgs particle is the simplest possible Higgs particle. The Higgs particle may be more complicated, and/or there may be multiple types of Higgs particles. But right now, at the Tevatron and LHC, the focus is on the following question: is the Higgs particle the one predicted by the Standard Model, and if so, what is its mass?

For years we’ve known that the Standard Model Higgs particle can only have a mass above 115 GeV (the limit comes from a previous collider called LEP II) and below about 800 GeV (the limit comes from a breakdown in the mathematics of the Standard Model.)

The Tevatron experiments CDF and DZero had managed to rule out a chunk of the Higgs mass range around 160 GeV, and updated that result this morning with some improvements, excluding about 155-175. Then came the ATLAS and CMS talks. I thought ATLAS and CMS would do a bit better than the Tevatron experiments. But I was completely wrong.

Starting late this morning and ending by mid-afternoon, I watched in some astonishment as step by step the LHC experiments ruled out most [and probably all-- we'll find out in a month at most when they combine their results] of the Higgs mass region between about 150 GeV and 450 GeV. This represents a giant leap forward, a spectacular end of the Tevatron’s reign, and very big news. The entire intermediate range for the Higgs mass may be gone, eliminated from our collective discussion in one short hour.

But that wasn’t the end of it, by a long shot. When ATLAS showed that they had an excess of events in their search for a Higgs decaying to 2 W particles, of a type not inconsistent with a Standard Model Higgs with mass in the 120-150 GeV range, I sat up a little straighter in my chair. We all waited to find out what CMS would say. And then CMS showed almost the same thing, though with smaller statistical significance. It could have been the reverse, as it has been so many times before; so often when one experiment sees something unusual, its sibling sees something normal, and we go back to our regular business. Not this time.

It was no ordinary day.

But what does it mean? No one knows. We all know of the following reasonable possibilities:

■Perhaps both experiments have had a statistical fluctuation which by chance is of a similar type.
■Perhaps both ATLAS and CMS are relying on theoretical calculations that are in some way inaccurate, or are using them in a way that makes them unreliable, thereby causing an apparent excess of events.
■Perhaps both experiments are not modeling one of their backgrounds correctly, and this is leading to an apparent excess of events.
■Perhaps there really is a Standard Model Higgs particle, or something similar, in the mass range of 120-150 GeV.
How can we tell the difference?

The first option can be addressed with more data; statistical fluctuations will tend to average out in larger data sets. Roughly 4 times the data would be good, and we should have that by fall.

To exclude the second or third is very tricky. The second requires a long discussion involving the experimentalists who did the search for the Higgs decaying to two W particles, and the theorists who are expert in the relevant calculations. I don’t know how long it would take to figure this out; months, perhaps. The third is hard too. Estimating backgrounds can be very tough. This certainly will require many experimental cross-checks. Many of these checks were already done before this data could be presented; many more will be possible with more data.

But it may actually be possible to rule in the fourth possibility, making it unnecessary to rule out the other options.. With more data one should be able to see Higgs particles decaying in ways that are rare but much less subtle to detect. I am thinking of Higgs decays to two Z particles or to two photons. Such decays, when gathered together, can give sharp peaks on a smooth background in data plots. In such cases, precise understanding of the background does not require great theoretical accuracy.

In a few months the experimentalists at ATLAS and CMS should have enough data to be able to straighten this out. I expect by somewhere between December and March we will have a definitive answer as to whether today’s events in Grenoble were truly historic, or merely a major milestone.

A last word of caution. It should still be viewed as unlikely that we have observed the first sign of the Higgs particle. Remember, most hints of new phenomena disappear with more data. But still, while it is true that few hints are followed by discoveries, it is also true that every discovery begins with a hint.

Re: World's Largest Atom Smasher May Have Detected 'God Particle' (the Higgs boson)

3:00 pm Kyle Cranmer of New York University presents the summary of the ATLAS search for the Standard Model Higgs thus far. This involves combining many different search strategies. Excludes 155-190 GeV at 95% confidence level, beats the Tevatron experiments, which are now basically obsolete for this search. First exclusion at high mass: 285-400 GeV now ruled out. Interesting excesses possible 120-145 GeV at 2.8 standard deviations. Particle physicists insist on 5 standard deviations for a convincing story…

[WARNING: see below for the meaning of this 2.8!!!]

3:20 pm Andrey Korytov presents the summary of the CMS search for the Standard Model Higgs thus far. Again 6 different studies combined together. 95% exclusion ranges are 149-206, 300-440, and much of the region from 200-300. 90% exclusion 145-480. Interesting excesses possible 120-145 but statistical significance hard to evaluate at this time. (but somewhat smaller excess than ATLAS sees.)

Whew! Exciting… but far too early to be sure this is anything interesting. A very small number of events are driving these effects, [Looking again at the data, that's not really correct --- for ATLAS there are more like 20 events over a background of maybe 100 (will get these numbers more precisely ASAP), for CMS something similar] and this might just be tricks of statistics. And measuring or calculating the background to the most important search, the one that looks for Higgs decaying to W’s, is very tough. Mistakes could easily be made here. But the fact that both experiments see the same thing is very provocative. This is certainly something we’ll be watching over the next couple of months.

By the way, one other possible interpretation would be that a Higgs particle is in the excluded range of 150-180 GeV, but with a smaller production rate than predicted by the Standard Model.

UPDATE: Been talking to Kyle Cranmer in more detail. Apparently the two LHC experiments did not share their info before today, so the similar hint of signal was revealed in today’s talks. And the attempt to combine their results starts now, and probably will not be done in time for this conference’s conclusion. Instead it may not be shown until late August at the next major conference… but this is speculation at this point.

We discussed what 2.8 standard deviations in ATLAS’s result means, precisely. [caution: technical stuff ahead] This is before the look elsewhere effect. It is the probability that given a strategy used for a particular mass (144 GeV I think) that there would have been this large a deviation just from background fluctuations.

A different question — and perhaps a more salient one — is what the probability would be that backgrounds in all the searches could have fluctuated in such a way that a deviation from expectation this large would have shown up. The probability quoted is 8 percent. That’s not so small. So in this sense, the 2.8 standard deviations number from ATLAS is somewhat of an overstatement of the unlikelihood of this happening in the data by accident.

What gives the result more weight is that both ATLAS and CMS see something similar.

And it has also been pointed out that both DZero and CDF show minor excesses in the same Higgs mass region.

Stay tuned. Within a few months we will know whether this is the real thing or not.

Re: World's Largest Atom Smasher May Have Detected 'God Particle' (the Higgs boson)

I'll have to look through the articles and stuff... I don't follow the energies of what's required for this stuff. All the news is either fucking stupid (black holes and dragons) or overly technical and esoteric.

Should ask Nathan over at that other place, too. He's moar in the loop.